There are basically two modes of operation for laser oscillators, the pulsed mode and the continuous wave (CW) mode. In the CW mode, the laser oscillator outputs a continuous power coherent light beam while the pulsed mode outputs a single burst of coherent light. The design of the resonant cavity and control mechanisms varies somewhat between these techniques. In the pulsed mode laser oscillator the output power can be enormously increased and the pulse duration correspondingly decreased by a technique known as Q-switching. Q-switching allows the resonator cavity of the laser oscillator to go from a high loss state to a low loss state and provides an element of control for the laser. In order to Q-switch a pulsed laser, a very high population inversion must be allowed to build up in the active medium prior to generation of an oscillatory mode. Q-switching involves making the resonator losses sufficiently high to prevent oscillation during a portion of the pump cycle, thus attaining a strong inversion, and then rapidly making the losses very small so that a large amplification is suddenly realized. The large amount of available energy stored in the laser medium is then discharged explosively within a few nanoseconds. The energy in the pulse is limited to the amount of energy that can be stored in the fully excited medium.
As an example, an ordinary laser might generate 100 mJ over a time interval of 100 microseconds for a peak power (averaged over this time interval) of 1000 W. The same laser Q-switched might emit 80 mJ in a single 10 nanosecond pulse for a peak power of 8 mW. The term Q-switching is used by analogy to the Q of an electric circuit. By lowering the Q of the optical cavity, the laser cannot oscillate, and a large inverted population builds up. When the cavity Q is restored, a single giant pulse can be generated.
In a non-Q-switched laser, each active line is cycled between energy levels several times during the typical pulse duration of a few hundred microseconds. Thus, considerably more total energy is emitted in this mode of operation than in the Q-switch mode in which the pulse duration is too short for de-excited ions to be repumped.
Four main types of Q-switching that have been employed in the prior art are the electro-optical, the rotating prism or mirror, the acousto-optical, and the saturable absorber. These techniques are further explained in Fink, et al, "Electronic Engineers Handbook", (1st ed., 1975) McGraw-Hill, page 1119, and Skolnik, "Radar Handbook", (1970), McGraw-Hill, page 37-30. In addition, a technique of controlling Q-switching in a pulsed laser is described in U.S. Pat. No. Re. 29,421, for a system that selectively varies the threshold of oscillation to select the gain of the laser cavity. The amount of power in the laser output is proportional to the amount above the threshold that the population inversion had reached prior to Q-switching. By varying the ratio of the oscillation threshold to the power stored in the laser medium, successive pulses can be equilibrated.
In a CW laser, as compared to the Q-switched laser, the laser medium is continually pumped to maintain a population inversion above the threshold in order to sustain an oscillation. By adjusting the reflectivity of the end mirrors in the resonant cavity, the round trip loss can be reduced to provide a higher laser output. This is essentially a method for adjusting the Q of the resonant cavity rather than switching it. As long as the laser medium is designed to accommodate a population inversion sufficient to maintain power to the coherent light beam, the resonant cavity will continue to oscillate. Techniques similar to those used in pulsed lasers can be utilized in the CW lasers to initiate oscillation.
In scanning lasers it is desirable to switch the position of the coherent light beam within the resonant cavity. This results in an unused portion of the laser medium being turned on, or Q-switched for each new beam position selected. As the output of the scanning laser is scanned across the field of view, the scanning rate can be as high as 10.sup.5 beam positions per second. This requires portions of the laser medium to make a transition from the high loss, low Q state to the low loss, high Q state for each beam position. As the laser medium is switched to a new beam position, it always oscillates in a mode that provides the lowest round trip loss, or highest Q, in the resonant cavity. When the new beam position is initially turned on with the use of a Q-switch, the resonant cavity changes from a high loss, low Q state, to a low loss, high Q state. During this transition from the high loss to the low loss state, there is a potential for the laser oscillator to lock into a spurious mode. This is due to the very high small signal gain in the laser medium when the laser cavity is in the high loss state with the Q-switch off.
To reduce the probability for spurious oscillations during switching in a scanning laser requires a Q-switching method that reduces the small signal gain of the laser medium when the Q-switch is off. This in turn reduces the potential for spurious mode oscillations when the cavity makes the transition from a high loss to a low loss state.